Hi, _M_check_len is used in vector reallocations. It computes __n + __s but does checking for case that (__n + __s) * sizeof (Tp) would overflow ptrdiff_t. Since we know that __s is a size of already allocated memory block if __n is not too large, this will never happen on 64bit systems since memory is not that large. This patch adds __builtin_constant_p checks for this case. This size of fully inlined push_back function that is critical for loops that are controlled by std::vector based stack.
With the patch to optimize std::max and to handle SRA candidates, we fully now inline push_back with -O3 (not with -O2), however there are still quite few silly things for example: // _78 is original size of the allocated vector. _76 = stack$_M_end_of_storage_177 - _142; _77 = _76 /[ex] 8; _78 = (long unsigned int) _77; _79 = MAX_EXPR <_78, 1>; _80 = _78 + _79; // this is result of _M_check_len doubling the allocated vector size. if (_80 != 0) // result will always be non-zero. goto <bb 7>; [54.67%] else goto <bb 13>; [45.33%] <bb 7> [local count: 30795011]: if (_80 > 1152921504606846975) // doubling succesfully allocated memmory will never get so large. goto <bb 8>; [10.00%] else goto <bb 11>; [90.00%] <bb 8> [local count: 3079501]: if (_80 > 2305843009213693951) // I wonder if we really want to have two different throws goto <bb 9>; [50.00%] else goto <bb 10>; [50.00%] <bb 9> [local count: 1539750]: std::__throw_bad_array_new_length (); <bb 10> [local count: 1539750]: std::__throw_bad_alloc (); <bb 11> [local count: 27715510]: _108 = _80 * 8; _109 = operator new (_108); Maybe we want to add assumption that result of the function is never greater than max_size to get rid of the two checks above. However this will still be recongized only after inlining and will continue confusing inliner heuristics. Bootstrapped/regtested x86_64-linux. I am not too familiar with libstdc++ internals, so would welcome comments and ideas. libstdc++-v3/ChangeLog: PR tree-optimization/110287 * include/bits/stl_vector.h: Optimize _M_check_len for constantly sized types and allocations. diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h index 70ced3d101f..3ad59fe3e2b 100644 --- a/libstdc++-v3/include/bits/stl_vector.h +++ b/libstdc++-v3/include/bits/stl_vector.h @@ -1895,11 +1895,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER size_type _M_check_len(size_type __n, const char* __s) const { - if (max_size() - size() < __n) - __throw_length_error(__N(__s)); + // On 64bit systems vectors of small sizes can not + // reach overflow by growing by small sizes; before + // this happens, we will run out of memory. + if (__builtin_constant_p (sizeof (_Tp)) + && __builtin_constant_p (__n) + && sizeof (ptrdiff_t) >= 8 + && __n < max_size () / 2) + return size() + (std::max)(size(), __n); + else + { + if (max_size() - size() < __n) + __throw_length_error(__N(__s)); - const size_type __len = size() + (std::max)(size(), __n); - return (__len < size() || __len > max_size()) ? max_size() : __len; + const size_type __len = size() + (std::max)(size(), __n); + return (__len < size() || __len > max_size()) ? max_size() : __len; + } } // Called by constructors to check initial size.